Radio frequency (RF) transmitters and receivers can be found in numerous applications, particularly in the field of wireless communications and radar sensors. In the automotive sector, there is an increasing demand for radar sensors used in so-called “adaptive cruise control” (ACC) or “radar cruise control” systems. Such systems may be used to automatically adjust the speed of an automobile so as to maintain a safe distance from other automobiles (and from other objects as well as from pedestrians) ahead.
Modern radar systems make use of highly integrated RF circuits, which may incorporate all core functions of an RF front-end of a radar transceiver in one single package (single chip radar transceiver), which is often referred to as MMIC (monolithic microwave integrated circuit). Such RF front-ends usually include, inter alia, a voltage controlled oscillator (VCO), power amplifiers (PA), directional couplers, mixers, and analog-to-digital converters (ADC), as well as respective control circuitry for controlling and monitoring of the RF front-end. Radar applications used in automobiles are subject to various standards concerning road traffic safety, for example the functional safety standard ISO 26262 titled “Road vehicles—Functional safety”. To ensure the functional safety of a radar sensor and/or to comply with statutory regulations, the RF front-end should operate with well-defined operation parameters.
For example, the RF front-end should provide an RF output signal (e.g. the transmit signal to be supplied to one or more antennas) with a defined output power in order to achieve a desired sensor performance. Furthermore, a maximum output power should not be exceeded during operation of the RF front-end in order to comply with statutory regulations. Moreover, the output power of the transmit signal should also not be lower than a minimum output power in order to ensure a reliable operation of the radar sensor. To comply with functional safety standards the MMIC should have the ability to detect malfunctions of the radar sensor, for example, when the RF output power is not within specified limits. Thus, there is a need to measure the power in an RF signal path in the RF front-end. However, available power sensing circuits (power sensors) have a poor accuracy and thus comparably broad guard bands are currently taken into account in the design of RF front-ends and during production tests in order to ensure to not leave the desired range of operation. Broad guard bands result in a correspondingly high number of deficient products and respective yield loss. Therefore, there is a need to improve accuracy of power sensing in integrated RF front-ends.